Summary:
The mineral deposits along the Carlin Trend form a suite of deposits known as Carlin type deposits.
Carlin mineral systems are a unique class of epigenetic ore deposits with enigmatic origins. They are unique due to the size and scale of deposits that formed exclusively within NE Nevada during the middle-Eocene period (42-35 Ma), establishing this area as a world-class gold complex. Carlin mineral systems are enigmatic due to their unknown, distal source of gold, as well as deposit characteristics and hydrothermal conditions that culminate to being atypical of most documented mineral systems.
South Arturo is a regionally distal Carlin-type deposit located in far north of the Carlin Trend that forms three structurally discrete ore bodies; the west, east and north. The western ore body (Phase 1) has an active open pit operation named South Arturo OP, which contains Mineral Resources and Mineral Reserves. Phase 1 was historically mined as both an open pit and underground. The eastern ore body (Phase 2) is an active underground operation named South Arturo UG (locally known as El Niño), which contains Mineral Resources and Mineral Reserves. Phase 2 was previously mined as an open pit.
South Arturo is a regionally distal Carfin-type deposit characterized by prolate mineralization geometries that are dominantly structurally controlled. Stratabound mineralization is apparent but far less continuous than similar deposits on the Carlin Trend. This likely reflects relatively lower hydrothermal fluid-rock interaction coupled with less favorable host rocks where planar faulting and fold features were the most permeable traps. Thus, mineralization orientations are highly variable due to the locally complex structural controls.
Mineralization is primarily hosted within the shelf-facies Rodeo Creek Formation. Discontinuous zones of mineralization occur within the allochthonous sequence of Vinini Formation but are minor. The platform-facies Bootstrap limestone is not a major host to mineralization but plays an important chemical and rheological competency contrast focusing fluid flow along its contact with Rodeo Creek Formation. This lithological contact is highly attenuated by pre-mineral deformation causing significant fault and dissolution breccias forming a highly permeable pathway.
The complex deformation history resulted in two oppositely plunging asymmetric anticlines. The intersection lineation of the two axial planes steeply plunges to the NW, which is the dominant control on the prolate mineralization geometries hosted within favorable Rodeo Creek sub-units of Phase 1. Conversely, Phase 2 mineralization is hosted within the brecciated contact of Rodeo Creek and Bootstrap, of which the widest zones of high-grade gold focuses in the apex of the local NE-trending anticlinal axial plane.
Mineralization also focuses in discrete dissolution breccia within high-angle normal faults that activate on the steep limb of the two regional anticlines, with highest grades focused in the Rodeo Creek Formation. Jurassic and Eocene dikes locally trend NW, and host volumetrically minor mineralization. All mineralization is associated with variable intensities of decalcification and argillization alteration. Mineralization occurs as oxide, sulfide refractory and carbonaceous refractory.
The South Arturo gold-silver deposits can be divided into five mineralized areas. These areas are the South Arturo, West Button Hill, Southwest Dee pit, Deep North, and Hinge. The majority of these deposits are classified as “Meikle Type” breccia hosted Carlin type deposits. A complex set of breccias occur at the upper contact of the Bootstrap limestone. These breccias can be generalized into four basic types: silicified heterolithic breccias, silica-Sulfide breccia, dolomite breccia, and cavity- fill breccias. While mineralization is widespread throughout the South Arturo Mine area, localized high-grade gold mineralization drives the economics and mine locations.
The dimensions and orientations of the modelled mineralized domains for South Arturo deposits:
Length - 520 – 1,500 m
Width - 70 – 550 m
True Thickness - 25 – 170 m
Approximate Strike Direction - NNW.
South Arturo
The northern extent of South Arturo mineralization lies approximately 200 feet southeast of the Dee pit and under 600 feet of waste rock. An overall north-south orientation to mineralization is inferred from the grade thickness contours, which define an area 1,700 feet in a north-south direction by 300 feet to 350 feet wide in an east-west direction. The mineralization and Tertiary contact dip 15° to 20° to the south. Drilling has shown that rocks are oxidized to a depth of up to 2,000 feet. Paleozoic rocks host the mineralization mainly in multi-stage, multi-lithic breccias with gold values ranging from 0.006 opt Au. These breccias are commonly formed by karsting or dissolution of carbonate rock and subsequent collapse and cavity fill. In general, decalcification is followed by weak to strong silicification with local argillization. Silver to gold ratios are generally 1:1 at grades of greater than 0.06 opt Au but increase to 5:1 at lower gold grade values.
Dee Deep North
Dee Deep North is a north-northeast trending pod of mineralization that plunges slightly north and is approximately 600 feet long, 150 feet wide, and 150 feet thick. The majority of high-grade refractory mineralization is in silica-Sulfide breccia within a flat to west-dipping silicified, multilithic breccia body above the Bootstrap limestone between 4,900 FASL and 5,100 FASL. The principal controls are the north-northeast trending high angle EB fault and southwest dipping low-angle structures.
Southwest Dee
The Southwest Dee pit mineralization is along the north-northeast trending, west dipping Dee Fault Zone. The mineralization is carbonaceous, partially oxidized, variably silicified mudstone/siltstone breccia approximately 300 feet in a north-south strike length, 100 feet wide, and 150 feet in thickness. The mineralization sits between 4,900 FASL and 5,100 FASL, with a small portion exposed along the southwest high wall at the bottom of the Dee pit.
West Button Hill
The West Button Hill mineralization trends north-northeast for over 2,000 feet in strike length, in pods that vary up to 600 feet wide and 50 feet thick. The majority of high-grade refractory mineralization is in the lower Rodeo Creek Formation and multi-lithic breccias above the Bootstrap limestone. The principal controls are the north-northeast and north-south trending high-angle structures and favorable host rocks. The mineralization has been shown to extend 1,230 feet below pre-mining surface elevation, as it is offset on the northeast striking, down-to-the-east Tara West fault.
Hinge
The Hinge zone is a north-south striking zone that lies between the Arturo zone to the south west and West Button Hill to the northeast and is due east relative to the existing Dee pit. It is approximately 1,400 feet long and up to approximately 300 feet wide, lying between elevations of 4,750 FASL and 5,250 FASL at depths from 330 feet to 900 feet below surface. Mineralization is hosted in the lower portion of the Rodeo Creek Formation and silicified breccias of the Basal Rodeo Creek and Popovich Upper Mud units. Breccia bodies drape the Bootstrap limestone. Mineralization in the Hinge zone is controlled by the Hinge fault, a steeply east dipping northsouth structure that appears to be a northerly extension of faults in Newmont’s Bootstrap pit to the south. Intersecting faults that influence mineralization have not been clearly identified. Much of the mineralization is partially to completely oxidized, even in the more deeply buried zones.